1. Field of the Invention
The present invention relates to an electron source substrate using electron-emitting devices, a production method thereof, and image forming apparatus using the electron source substrate.
2. Related Background Art
As conventional image forming apparatus making use of electron-emitting devices, there are known flat electron beam display panels in which an electron source substrate with a number of cold cathode electron-emitting devices therein and an anode substrate with a transparent electrode and a fluorescent material are opposed to each other in parallel and in which the interior is evacuated into vacuum. Among such image forming apparatus, one using field emission electron-emitting devices is disclosed, for example, in I. Brodie, “Advanced technology: flat cold-cathode CRTs,” Information Display, 1/89, 17 (1989). The field emission electron-emitting devices with a pair of electrodes on a substrate surface are disclosed in Japanese Patent Applications Laid-Open No. 63-274047 and No. 63-274048, U.S. Pat. No. 4,904,895, and so on.
The apparatus using surface conduction electron-emitting devices is disclosed, for example, in U.S. Pat. No. 5,066,883 and other documents. The flat electron beam display panels can be substantiated in lighter-weight and larger-screen structure than the cathode ray tube (CRT) display apparatus commonly used at present, and can provide images with higher luminance and higher quality than the other flat display panels such as flat display panels making use of liquid crystals, plasma displays, electroluminescent displays, and so on.
Particularly, the surface conduction electron-emitting devices are simple in structure and easy in production and are advantageous in that an electron source substrate with a number of devices arrayed over a large area can be fabricated without need for going through complicated production steps taking advantage of the photolithography technologies, whereas the field emission electron-emitting devices have to be fabricated through such steps.
FIG. 10 shows an example of the electron source substrate using the surface conduction electron-emitting devices, which is disclosed in Japanese Patent Application Laid-Open No. 06-342636. FIG. 10 is a plan view showing part of the electron source. In FIG. 10, reference numeral 1 designates a base, 2 and 3 device electrodes, 4 an electro-conductive thin film having an electron-emitting region in each device, and 5 the electron-emitting region in each device. The device electrodes 2, 3 are coupled to lower wiring lines 6 and to upper wiring lines 7, respectively, and the lower wiring lines 6 and upper wiring lines 7 are electrically insulated from each other by interlayer dielectric layers 8. Numeral 101 indicates one of surface conduction electron-emitting devices.
Predetermined voltages are successively applied as scanning signals and as information signals to the upper wires 7 and to the lower wires 6, respectively, in the matrix arrangement, whereby the predetermined electron-emitting devices located at intersections of the matrix can be selectively driven.
This matrix-arranged electron source substrate can be fabricated by relatively simple photolithography technologies, but it is preferable to employ print techniques for formation of larger substrates. Particularly, concerning the upper wiring lines to which the scanning signals are applied, the amount of electric current flowing through the wiring increases with increase in the number of devices connected per line, so as to result in a voltage drop due to wiring resistance; therefore, it is preferable to form thick films of wiring so as to reduce the resistance as much as possible.
Japanese Patent Application Laid-Open No. 08-180797 and others disclose production methods of forming the wiring and interlayer dielectric layers by screen printing. As for the other members, for example, Japanese Patent Application Laid-Open No. 09-17333 and others disclose production methods of forming the device electrodes by offset printing or the like, and Japanese Patent Application Laid-Open No. 09-69334 and others disclose production methods of forming the electroconductive thin films by the ink jet method. By employing these printing techniques, it is feasible to fabricate the electron source substrate of large area readily.
Besides the above-cited documents, there are various reports on the surface conduction electron-emitting devices; for example, those using a thin film of SnO2 [M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965)] and [G. Dittmer: “Thin Solid Films,” 9, 317 (1972)]; those using a thin film of In2O3/SnO2 [M. Hartwell and C. G. Fonstad: “IEEE Trans. ED Conf.,” 51 9 (1975)]; those using a thin film of carbon [Hisashi Araki et al.: “Vacuum, Vol. 26, No. 1, p22 (1983)], and so on. For example, Japanese Patent Application Laid-Open No. 02-56822 discloses the surface conduction electron-emitting devices using a metal microparticle film of palladium oxide or the like.
In fabrication of the surface conduction electron-emitting devices, it is common practice to form the electron-emitting regions 5 by energization operation, called forming, on the electroconductive thin films 4. The forming is an operation of placing a dc voltage or a very slowly increasing voltage, e.g., approximately 1 V/min, at the both ends of each electroconductive thin film 4 to locally break, deform, or modify the electroconductive thin film 4, thereby forming a fissure.
After completion of the forming operation, a voltage is applied to the electroconductive thin film 4 to let an electric current flow through the device, whereupon electrons are emitted from the vicinity of the fissure. A portion emitting electrons at this time is called the electron-emitting region 5.
Furthermore, it is feasible to achieve better electron emission, by effecting a process called an activation operation on the devices after the forming, for example, as disclosed in Japanese Patent Application Laid-Open No. 07-235255. The activation step can be performed by repeatedly applying a pulse voltage to the devices, similarly as in the forming operation, under an atmosphere containing a gas of an organic substance, whereby carbon or carbon compounds are deposited from the organic substance existing in the atmosphere, onto the devices. The activation operation extremely increases the device current If and the emission current Ie.
The surface conduction electron-emitting devices fabricated through these operations have adequate electron emission characteristics as electron sources applicable to the image forming apparatus, for example, such as the flat panel displays and others.
By fabricating the large-area electron source substrate comprised of the surface conduction electron-emitting devices by the print techniques as described above, it is thus feasible to realize a large-area image forming apparatus, e.g., a large-screen flat panel display.
In the case where the large-area electron source substrate is constructed of the electron-emitting devices with sufficient electron emission amount, lifetime, and stability, there arises a problem as discussed below, however.
In the electron source in which the wiring electrodes connected the electron-emitting devices (e.g., the surface conduction electron-emitting devices) each with the electron-emitting region between a pair of device electrodes provided on the surface of the substrate, the device electrodes and the wiring lines were often made of different compositions from the demands for production cost, improvement in electron emission characteristics, and so on. The term “different compositions” herein means both (1) materials different from each other and (2) materials of identical elements but different composition ratios.
For example, (1) includes a case where the wires are made of silver (Ag) and the device electrodes of platinum (Pt), a case where the wires are made of ruthenium oxide (RuO2) and the device electrodes of ruthenium (Ru), and so on. Alloys of different compositions (e.g., a case where the wires are made of an alloy of gold and iridium (A—Ir) and the device electrodes of an alloy of gold and indium (Au—In)) also fall under the category of the different compositions. (2) includes a case where the device electrodes are made of a solder alloy of tin (Sn) and lead (Pb) at a composition ratio of Sn:Pb=7:3 and the wires of a solder alloy thereof at a composition ratio of Sn:Pb=6:4.
In the case of the device electrodes and the wires being made of different compositions as described, the process through the steps of high-temperature processing and the like sometimes caused the wiring material to migrate through the interface between the device electrodes and the substrate and reach the electron-emitting regions, so as to induce unexpected change in the electron-emitting regions, thus posing the problem of change in the electron emission characteristics. The Inventor found that this phenomenon was apt to occur, particularly, in the case where a processed film or the like for preventing diffusion of the substrate material was provided on the surface of the electron source substrate. This will be detailed below with a specific configuration example.
For producing the large-area electron source substrate at low cost, it is necessary to reduce the cost of the members used, and soda lime glass is preferably used for the base of the substrate. In the case of the electron-emitting devices each with a pair of devices electrodes on the substrate surface and the electron-emitting region between the device electrodes, typified by the surface conduction electron-emitting devices and the lateral field emission devices disclosed in aforementioned U.S. Pat. No. 4,904,895, because the electron-emitting regions are formed in contact with the substrate surface, heat and electric fields generated during driving of the electron-emitting devices are also transmitted to the surface of soda lime glass, so as to readily induce thermal deformation of the substrate, migration of sodium ions, precipitation of metal sodium and sodium compounds, and so on. The deformation of the substrate near the electron-emitting devices would cause change of the device structure and the precipitation of sodium would also change the electrical property as well as the structure, thus causing variation and degradation of the electron emission characteristics.
For this reason, for example, in the case of the surface conduction electron-emitting devices, it is desirable to form a coating layer of a material containing the main component of silicon dioxide on the soda lime glass surface and form the surface conduction electron-emitting devices thereon, as disclosed in Japanese Patent Application Laid-Open No. 01-279538. Particularly, when a silica layer or a phosphorus-doped silica (PSG) layer approximately 500 nm or more thick is formed as this coating layer, the coating layer makes it harder for the heat and electric fields in driving of the surface conduction electron-emitting devices to be transmitted to the soda lime glass base and thus can serve as an adequate sodium diffusion preventing layer. As disclosed in Japanese Patent Application Laid-Open No. 2000-215789, it is also possible to form a layer containing an electroconductive oxide, on the soda lime glass surface, further provide a layer containing the main component of silicon dioxide on the surface thereof, and form the surface conduction electron-emitting devices thereon. This coating layer of the two-layer structure also functions better as a sodium diffusion preventing layer to suppress the diffusion of sodium due to the heat and electric fields.
This sodium diffusion preventing layer does not block only the sodium diffusion from the base but also blocks the diffusion of metal placed on the sodium diffusion preventing layer, to the substrate, however. In the structure where the metal is hard to diffuse into the substrate, repetition of thermal treatment steps can result in diffusion of the metal in parallel to the substrate surface along the substrate surface and along the interface between the substrate and the other members.
In the electron source substrate of the configuration as shown in FIG. 10, this diffusion in parallel to the substrate surface occurs at locations where the wiring metal contacts the interface between the device electrodes and the substrate, and becomes more prominent in the case where the device electrodes are made of a metal, particularly, in the case where the device electrodes are made of a metal different from the wiring metal. If the wiring metal diffuses through the interface between the device electrodes and the substrate surface (or the surface of the sodium diffusion preventing layer), it will come to contact the electroconductive, thin films. If a further thermal treatment is carried out at this point or if an electric field in driving is applied, migration of the metal due to the heat or the electric field will result in mixing the wiring metal with the electroconductive thin films and it will become hard to maintain the expected electron emission characteristics of the electron-emitting devices, thus inducing the degradation and variation of characteristics. Accordingly, it was necessary to control the diffusion through the interface of the wiring metal as low as possible.
A solution to this problem heretofore was a method of forming the electron-emitting devices, using an electroconductive oxide for the device electrodes, as disclosed in Japanese Patent Application Laid-Open No. 2000-243327. However, if discharge occurs during driving of the devices, influence thereof will be significant. There were cases where the discharge did not affect only a discharging device but also affected the devices around it, so as to increase the size of defects in the substrate.